EL Display Device and Method for Manufacturing the Same

ABSTRACT

A plurality of pixels are arranged on the substrate. Each of the pixels is provided with an EL element which utilizes as a cathode a pixel electrode connected to a current control TFT. On a counter substrate, a light shielding film, a first color filter having a first color and a second color filter having a second color are provided. The second color is different from the first color.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an EL (Electro-luminescence) displaydevice formed by fabricating a semiconductor device (a device utilizinga semiconductor thin film; typically a thin film transistor) on asubstrate, and an electronic device having such an EL display device asa display portion.

2. Description of the Related Art

A technique for forming a thin film transistor (hereinafter referred toas the TFT) on a substrate has been significantly improved in thesedays, and development for application thereof to an active matrix typedisplay device has continued. In particular, a TFT utilizing apolysilicon film has a field effect mobility higher than that obtainablein a TFT utilizing a conventional amorphous silicon film, therebyrealizing an operation at higher speed. Thus, it becomes possible tocontrol pixels by a driving circuit formed on the identical substrate onwhich the pixels are formed, which is different from the conventionalcase in which the pixels are controlled by a driving circuit formed inthe outside of the substrate.

Such an active matrix type display device has drawn much attentionthereto since the device can realize various advantages, such asreduction in manufacturing cost, down-sizing of a display device, animproved yield, a reduced throughput or the like, by fabricating variouscircuits and devices on one and the same substrate.

In an active matrix EL display device, each pixel is provided with aswitching device made of a TFT, and a driving device for controlling acurrent is activated by the switching device to cause an EL layer (morestrictly speaking, a light emitting layer) to emit light. The EL displaydevice is disclosed, for example, in Japanese Laid-open PatentPublication No. Hei 10-189252.

Thus, the present invention is intended to provide an inexpensive ELdisplay device capable of displaying an image with high definition.Furthermore, the present invention is also intended to provide anelectronic device having a high recognizability of a display portion byutilizing such an EL display device as the display portion.

SUMMARY OF THE INVENTION

The present invention will be described with reference to FIG. 1. InFIG. 1, reference numeral 101 denotes a substrate having an insulatingsurface. As the substrate 101, an insulating substrate such as a quartzsubstrate can be used. Alternatively, various kinds of substrate, suchas a glass substrate, a semiconductor substrate, a ceramic substrate, acrystallized substrate, a metal substrate, or a plastic substrate, canbe used by providing an insulating film on a surface thereof.

On the substrate 101, pixels 102 are formed. Although only three of thepixels are illustrated in FIG. 1, a higher number of pixels are actuallyformed in matrix. In addition, although one of these three pixels willbe described below, the other pixels have the same structure.

In each of the pixels 102, two TFTs are formed; one of them is aswitching TFT 103, and the other is a current control TFT 104. A drainof the switching TFT 103 is electrically connected to a gate of thecurrent control TFT 104. Furthermore, a drain of the current control TFT104 is electrically connected to a pixel electrode 105 (which in thiscase, also functions as a cathode of an EL element). The pixel 102 isthus formed.

Various wirings of the TFT as well as the pixel electrode can be formedof a metal film having a low resistivity. For example, an aluminum alloyfilm may be used for this purpose.

Following the fabrication of the pixel electrode 105, an insulatingcompound 106 (hereinbelow, referred to as the alkaline compound)including an alkali metal or an alkaline-earth metal over all of thepixel electrodes. Note that the outline of the alkaline compound isindicated by a dotted line in FIG. 1. This is because the alkalinecompound 106 has a thickness which is as thin as several nm, and it isnot known whether the compound 106 is formed as a layer or in anisland-shape.

As the alkaline compound, lithium fluoride (LiF), lithium oxide (Li₂O),barium fluoride (BaF₂), barium oxide (BaO), calcium fluoride (CaF₂),calcium oxide (CaO), strontium oxide (SrO), or cesium oxide (Cs₂O) canbe used. Since these are insulating materials, short-circuiting betweenthe pixel electrodes does not occur even when the alkaline compound 106is formed as a layer.

It is of course possible to use as a cathode a known conductive materialas an MgAg electrode. However, in this case, the cathode itself has tobe selectively formed or patterned into a certain shape in order toavoid short-circuiting between the pixel electrodes.

After the alkaline compound 106 is formed, an EL layer(Electro-luminescence layer) 107 is formed thereover. Although any knownmaterial and/or structure can be employed for the EL layer 107, amaterial capable of emitting white light is used in the presentinvention. As the structure, only a light emitting layer providing afield for recombination may be employed for the EL layer. If necessary,an electron injection layer, an electron transport layer, a holetransport layer, an electron blocking layer, a hole device layer, orwhole injection layer may be further layered. In the presentspecification, all of those layered intended to realize injection,transport or recombination of carriers are collectively referred to asthe EL layer.

As an organic material to be used as the EL layer 107, either alow-molecule type organic material or a polymer type (high-moleculetype) organic material can be used. However, it is desirable to use apolymer type organic material that can be formed by an easy formationtechnique such as a spin coat technique, a printing technique or thelike. The structure illustrated in FIG. 1 is of the color display schemein which an EL layer for emitting white light is combined with a colorfilter.

Alternatively, a color display scheme in which an EL layer for emittingblue or blue-green light is combined with fluorescent material(fluorescent color conversion layer; CCM), or another color displayscheme in which EL layers respectively corresponding to RGB are overlaidone upon another to provide color display, can also be employed.

Over the EL layer 107, a transparent conductive film is formed as ananode 108. As the transparent conductive film, a compound of indiumoxide and tin oxide (referred to as ITO), a compound of indium oxide andzinc oxide, tin oxide or zinc oxide can be used.

Over the anode 108, an insulating film as a passivation film 109 isprovided. As the passivation film 109, a silicon nitride film or asilicon oxynitride film (represented as SiOxNy) is preferably used.Although a silicon oxide film may be used, an insulating film with aslow an oxygen content as possible is preferred.

The substrate fabricated up to this stage is referred to as an activematrix substrate in the present application. More specifically, thesubstrate on which a TFT, a pixel electrode electrically connected tothe TFT, and an EL element (a capacitor made of a cathode, an EL layer,and an anode) utilizing the pixel electrode as the cathode are formed isreferred to as the active matrix substrate.

Furthermore, a counter substrate 110 is attached to the active matrixsubstrate with the EL element being interposed therebetween. The countersubstrate 110 is provided with a light shielding film 112 and colorfilters 113 a to 113 c.

At this situation, the light shielding film 112 is provided so that agap 111 formed between the pixel electrodes 105 is unseen from theviewing direction of an observer (i.e., from a direction normal to thecounter substrate.) More specifically, the light shielding film 112 isprovided to overlap (align with) the periphery of the pixel when viewedfrom the direction normal to the counter substrate. This is because thisportion is non-emitting portion, and furthermore, electric field becomescomplicated at the edge portion of the pixel electrode and thus lightcannot be emitted with a desired luminance or chromaticity.

More specifically, by providing the light shielding film 112 at theposition corresponding to the periphery (edge portion) of the pixelelectrode 105 and the gap 111, contour between the pixels can be madeclear. It can be also said that in the present invention, the lightshielding film 112 is provided at the position corresponding to theperiphery (edge portion) of the pixel because the contour of thepixel-electrode-corresponds to the contour of the pixel. Note that theposition corresponding to the periphery of the pixel refers to theposition aligned with the periphery of the pixel when viewed from theaforementioned direction which is normal to the counter substrate.

Among the color filters 113 a to 113 c, the color filter 113 a is theone for obtaining red light, the color filter 113 b is the one forobtaining green light, and the color filter 113 c is the one forobtaining blue light. These color filters are formed at positionsrespectively corresponding to the different pixels 102, and thus,different color of light can be obtained for the respective pixels. Intheory, this is the same as the color display scheme in a liquid crystaldisplay device which uses color filters. Note that the positioncorresponding to the pixel refers to the position overlapped (aligned)with the pixel when viewed from the aforementioned direction which isnormal to the counter substrate. More specifically, the color filters113 a to 113 c are provided so as to overlap the pixels respectivelycorresponding thereto when viewed from the direction normal to thecounter substrate.

Note that the color filter is a filter for improving the color purity oflight which has pawed therethrough by extracting light of a specificwavelength. Accordingly, in the case where the light component of thewavelength to be extracted is small, there may be disadvantages in whichthe light of that wavelength has extremely small luminance ordeteriorated color purity. Thus, although no limitation is imposed to anEL layer for emitting white light which can be used in the presentinvention, it is preferable that the spectrum of the emitted white lightincludes emission spectrums of red, green and blue light componentshaving as high purity as possible.

FIGS. 16A and 16B show typical x-y chromaticity diagrams of an EL layerto be used in the present invention. More specifically, FIG. 16A showsthe chromaticity coordinate of light emitted from a known polymer typeorganic material for emitting white light. In the known material, thered color emission with high color purity cannot be realized. Therefore,yellow light or orange light is used instead of red light. Accordingly,white color obtained by adhesive color mixing seems to slightly includegreen color or yellow color. In addition, the respective emissionspectrums of red light, green light, and blue light are so broad that italso becomes difficult to obtain monochromatic light having high puritywhen these light are mixed.

Accordingly, although sufficient color display can be realized even whenan organic material as represented in the chromaticity diagram in FIG.16A is used as an EL layer, it is now preferable to use as an EL layeran organic material as represented in the chromaticity diagram in FIG.16B in order to realize brighter color display with higher purity.

The organic material as represented in the chromaticity diagram in FIG.16B is an example in which an EL layer for emitting white light isformed by mixing organic materials capable of providing monochromaticlight with high purity. In order to obtain light emission spectrums ofred, green and blue colors having high color purity from a color filter,it is necessary to form an EL layer for emitting white light by mixingorganic materials respectively exhibiting light emission spectrums ofred, green and blue colors with high color purity. In addition, by usingmaterials capable of providing a spectrum not only with high colorpurity but also with a narrow half-peak width, white color with a sharpspectrum can be reproduced. With this kind of the EL layer for emittingwhite light, the present invention can display a further brighter colorimage.

Furthermore, the color filters 113 a to 113 c can contain, as a dryingagent, an oxide of an element in group I or II in periodic table, e.g.,barium oxide, calcium oxide, lithium oxide or the like. In this case, aresin film containing a drying agent and a pigment of red, green or bluecolor may be used as a color filter.

Note that although not illustrated herein, the counter substrate 110 isadhered to the active matrix substrate by means of a sealing agent, sothat a space designated with reference numeral 114 is a closed space.

As the counter substrate 110, it is necessary to use a transparentsubstrate so as not to prevent light from traveling. For example, aglass substrate, a quartz substrate, or a plastic substrate ispreferably used. In addition, as the light shielding film 112, a thinfilm capable of satisfactorily shielding light, e.g., a titanium film, aresin film including a black-colored pigment or carbon. Similarly to thecase of the above-mentioned color filters 113 a to 113 c, it isadvantageous to provide the light shielding film 112 containing, as adrying agent, an oxide of an element in group I or II in periodic table,e.g., barium oxide, calcium oxide, lithium oxide or the like.

The closed space 114 may be filled with inert gas (noble gas or nitrogengas), or with inert liquid. Alternatively, the closed space 114 may befilled with a transparent adhesive so as to adhere the whole surface ofthe substrate. Moreover, it is preferable to dispose a drying agent suchas barium oxide in the closed space 114. Since the EL layer 107 is veryvulnerable to water, it is highly desirable to prevent water fromentering the closed space 114.

In the EL display device having the above-mentioned construction inaccordance with the present invention, light emitted from the EL elementpasses through the counter substrate to be emitted toward an observer'seyes. Accordingly, the observer can recognize an image through thecounter substrate. In this situation, one of the features of the ELdisplay device in accordance with the present invention is that thelight shielding film 112 is disposed between the EL element and theobserver so as to conceal the gap 111 between the pixel electrodes 105.Thus, the contour between the pixels can be made clear, therebyresulting in an image display with high definition. This advantage canbe obtained due to the light shielding film 112 provided at the countersubstrate 110. When at least the light shielding film 112 is provided,this advantage can be obtained.

Furthermore, the light shielding film 112 and the color filters 113 a to113 c are disposed at the counter substrate 110, and the countersubstrate 110 also functions as a ceiling substrate for suppressingdeterioration of the EL element. When the light shielding film 112 andthe color filters 113 a to 113 c are disposed at the active matrixsubstrate, additional film-formation and patterning steps are required,whereas the number of fabrication steps for the active matrix substratecan be suppressed in the case where they are provided at the countersubstrate, although additional film-formation and patterning steps arerequired.

Furthermore, the structure in accordance with the present invention inwhich the counter substrate 110 is provided with the light shieldingfilm 112 and the color filters 113 a to 113 c and adhered to the activematrix substrate by means of the sealing agent has features common tothe structure of a liquid crystal display device. Accordingly, it ispossible to fabricate the EL display device of the present inventionwith most of an existing manufacturing line for liquid crystal displaydevices. Thus, an amount of equipment investment can be significantlyreduced, thereby resulting in a reduction in the total manufacturingcost.

Thus, in accordance with the present invention, an inexpensive ELdisplay device capable of displaying an image with high definition canbe obtained. Furthermore, the present invention can also provide anelectronic device having a high recognizability of a display portion byutilizing such an EL display device as the display portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view for illustrating a pixel portion of an EL displaydevice.

FIG. 2 is a view for illustrating the cross-sectional structure of apixel of an EL display device.

FIG. 3A is a view for illustrating the top structure of a pixel portionof an EL display device.

FIG. 3B is a view for illustrating the configuration of a pixel portionof an EL display device.

FIGS. 4A through 4E are views for illustrating the fabricating steps ofan active matrix type EL display device.

FIGS. 5A through 5D are views for illustrating the fabricating steps ofan active matrix type EL display device.

FIGS. 6A through 6C are views for illustrating the fabricating steps ofan active matrix type EL display device.

FIG. 7 is a view for illustrating the perspective appearance of an ELdisplay device.

FIG. 8 is a view for illustrating the circuit block configuration of anEL display device.

FIG. 9 is an enlarged view of a pixel of an EL display device.

FIG. 10 is a view of the structure of a sampling circuit of an ELdisplay device.

FIG. 11A is a top view for illustrating the appearance of an EL displaydevice:

FIG. 11B is a cross-sectional view for illustrating the appearance of anEL display device.

FIG. 12 is a view for illustrating the pixel structure of an EL displaydevice.

FIG. 13 is a view for illustrating the cross-sectional structure of apixel of an EL display device.

FIGS. 14A to 14F are views for respectively illustrating specificexamples of an electronic device.

FIGS. 15A and 15B are views for respectively illustrating specificexamples of an electronic device.

FIGS. 16A and 16B are diagrams for respectively showing the chromaticitycoordinates of organic materials.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Some embodiments of the present invention will be described withreference to FIGS. 2, 3A and 3B. FIG. 2 shows a cross-sectional view ofa pixel portion in an EL display device in accordance with the presentinvention. FIG. 3A shows a top view of the pixel portion, and FIG. 3Bshows the circuit configuration thereof. In an actual structure, pixelsare arranged in a plurality of lines to be in matrix, thereby forming apixel portion (image display portion). FIG. 2 illustrates atruss-sectional view taken along the line A-A′ in FIG. 3A. Accordingly,the same components are commonly designated by the same referencenumerals in both of the figures, and it will be advantageous forunderstanding the structure to make reference to both of the figures. Inaddition, the two pixels illustrated in the top view of FIG. 3A have thesame structure.

In FIG. 2, reference numeral 11 denotes a substrate, and 12 denotes abase insulating film (hereinafter referred to as the base film). As thesubstrate 11, a glass substrate, a glass ceramic substrate, a quartzsubstrate, a silicon substrate, a ceramic substrate, a metal substrate,or a plastic substrate (including a plastic film) can be used.

In addition, the base film 12 is especially advantageous for a substrateincluding mobile ions or a substrate having conductivity, but does notnecessarily have to be provided for a quartz substrate. As the base film12, an insulating film-containing silicon may be used. In the presentspecification, the “insulating film containing silicon” refers to aninsulating film containing silicon and oxygen or nitrogen at apredetermined ratio, and more specifically, a silicon oxide film, asilicon nitride film, or a silicon oxynitride film (represented asSiOxNy.)

It is advantageous to provide the base film 12 with a heat radiationfunction to dissipate heat generated in a TFT in order to prevent a TFTor an EL element from deteriorating. The heat radiation function can beprovided by any known material.

In this example, two TFTs are provided in one pixel. A TFT 201 functionsas a switching device (hereinafter referred to as the switching TFT),and a TFT 202 functions as a current controlling device for controllingan amount of current to flow through the EL element, (hereinafterreferred to as the current control TFT.) Both of the TFTs 201 and 202are made of the n-channel type TFT.

Since the n-channel type TFT has a field effect mobility higher thanthat of the p-channel type TFT, the n-channel type TFT can operate athigher speed and accept a large amount of current. Furthermore, acurrent of the same amount can flow through the n-channel type TFT ofsmaller size as compared to the p-channel type TFT. Accordingly, it ispreferable to use the n-channel type as the current control TFT sincethis results in an increased effective area of the display portion.

The p-channel type TFT has advantages, e.g., in which the injection ofhot carriers becomes hardly a problem and an OFF current value is small.Thus, it has been already reported the structures in which the p-channeltype TFT is used as the switching TFT or the current control TFT.However, in the present invention, the disadvantage-s in connection withthe injection of hot carriers and a small OFF current value can beovercome even in the n-channel type TFT by providing the structure withshifted LDD regions. Thus, it is another feature of the presentinvention in which all of the TFTs is in the pixel are made of then-channel type TFTs.

However, the present invention is not limited to the case where theswitching TFT and the current control TFT are made of the n-channel typeTFTs. It is possible to use the p-channel type TFT as both or either ofthe switching TFT and the current control TFT.

The switching TFT 201 is formed to have a source region 13, a drainregion 14, an active layer including LDD regions 15 a to 15 d and a highconcentration impurities region 16 and channel forming regions 17 a and17 b, a gate insulating film 18, gate electrodes 19 a and 19 b, a firstinterlayer insulating film 20, a source wiring 21, and a drain wiring22.

In addition, as shown in FIGS. 3A and 3B, the gate electrodes 19 a and19 b are electrically connected to each other by means of a gate wiring211 which is made of a different material (that has a lower resistivitythan the gate electrodes 19 a and 19 b), thereby forming a double-gatestructure. It is of course possible to employ, not only the double-gatestructure, but also the so-called multigate structure (a structureincluding an active layer which contains two or more channel formingregions connected in series) such as a triple-gate structure. Themultigate structure is significantly advantageous for decreasing of OFFcurrent value. In accordance with the present invention, a switchingdevice having a low OFF current value can be realized by providing theswitching device 201 in the pixel with the multigate structure.

In addition, the active layer is formed of a semiconductor film thatincludes a crystalline structure. This may be a single crystallinesemiconductor film, a polycrystalline semiconductor film, or amicrocrystalline semiconductor film. The gate insulating film 18 may beformed of an insulating film containing silicon. Furthermore, any kindof conductive films can be used as the gate electrode, the sourcewiring, or the drain wiring.

Furthermore, in the switching TFT 201, the LDD regions 15 a to 15 d aredisposed so as not overlap the gate electrodes 19 a and 19 b. Such astructure is significantly advantageous for reducing an OFF currentvalue.

For reducing the OFF current value, it is further preferable to providean offset region (which is made of a semiconductor layer having the samecomposition as the channel forming regions and a gate voltage is notapplied thereto) between the channel forming regions and the LDDregions. In addition, in the case of the multigate structure having twoor more gate electrodes, the high concentration impurities regiondisposed between the channel forming regions is effective for reducingthe OFF current value.

As set forth above, by employing the TFT having the multigate structureas the switching device 201 in the pixel, a switching device having asufficiently low OFF current value can be realized. Thus, a gate voltageof the current control TFT can be maintained for a sufficient period oftime (from the timing selected until the next timing selected) withoutproviding a capacitor as shown in FIG. 2 of Japanese Laid-Open PatentPublication No. Hei 10-189252.

Then, the current control TFT 202 is formed to have a source region 31,a drain region 32, an active layer including an LDD region 33 and achannel forming region 34, a gate insulating film 18, a gate electrode35, a first interlayer insulating film 20, a source wiring 36, and adrain wiring 37. Although the illustrated gate electrode 35 has thesingle gate structure, it may have the multi-gate structure.

As shown in FIG. 2, a drain of the switching TFT is connected to a gateof the current control TFT. More specifically, the gate electrode 35 iselectrically connected to the drain region 14 of the switching TFT 201through the drain wiring 22 (also referred to as the connecting wiring).Furthermore, the source wiring 36 is connected to a power supply line212.

The current control TFT 202 is a device intended to control an amount ofcurrent to be injected into the EL element 203. However, consideringpossible deterioration of the EL element, it is not preferable to allowa large amount of current to flow. Accordingly, in order to preventexcessive current from flowing through the current control TFT 202, thechannel length (L) thereof is preferably designed to be long. Desirably,the channel length (L) is design to be 0.5 to 2 μm (preferably, 1 to 1.5μm) long per pixel.

In view of the above-mentioned description, as shown in FIG. 9, thechannel length L1 (where L1=L1 a+L1 b) and the channel width W1 of theswitching TFT, and the channel length L2 and the channel width W2 of thecurrent control TFT are preferably set as follows: W1 is in the rangefrom 0.1 to 5 μm (typically, 0.5 to 2 μm); W2 is in the range from 0.5to 10 μm (typically, 2 to 5 μm); L1 is in the range firm 0.2 to 18 μm(typically, 2 to 15 μm); and L2 is in the range from 1 to 50 μm(typically, 10 to 30 μm). However, the present invention is not limitedto the above-mentioned values.

The length (width) of the LDD regions to be formed in the switching TFT201 is set in the range from 0.5 to 33 μm, typically in the range from2.0 to 2.5 μm.

The EL display device as shown in FIG. 2 has features in which the LDDregion 33 is provided between the drain region 32 and the channelforming region 34 in the current control TFT 202, and part of the LDDregion 33 overlaps the gate electrode 35 through the gate insulatingfilm 18 while the remaining portion does not.

The current control TFT 202 supplies a current to the EL element 203 forcausing it to emit light, and at the same time controls the suppliedamount of the current to realize gray scale display. For that purpose,it is necessary to implement countermeasure against deterioration duo tothe injection of hot carriers so as not to deterioration when thecurrent flows. Further, in the case where black color is displayed byturning off the current control TFT 202, a high OFF current valueprevents black color from being displayed in satisfactory condition,resulting in disadvantages such as reduced contrast. Accordingly, it isalso necessary suppress the OFF current value.

With respect to the deterioration due to the injection of hot carriers,it has been known that the structure with the LDD region overlapping thegate electrode is very effective. However, when the whole LDD regionoverlaps the gate electrode, the OFF current value increases. Thus, thepresent inventors overcome both the disadvantage related to the hotcarriers and the disadvantage related to the OFF current value byproviding a novel structure with the LDD regions being disposed inseries so as not to overlap the gate electrode.

In this case, the length of the LDD region overlapping the gateelectrode may be set to be in the range from 0.1 to 3 μm (preferably inthe range of 0.3 to 1.5 μm). If the overlap length is too long,parasitic capacitance is increased, whereas when the overlap length istoo short, hot carriers, cannot be sufficiently suppressed. Furthermore,a region of the LDD not overlapping the gate electrode may be set to bein the range from 1.0 to 33 μm (preferably in the range of 1.5 to 2.0μm). If this length is too long, a sufficient current cannot flow,whereas when this length is too short, the OFF current value cannot besufficiently reduced.

Furthermore, in the above structure, a parasitic capacitance isgenerated in the region where the gate electrode overlaps the LDDregion, and therefore, such an overlap region should not be providedbetween the source region 31 and the channel forming region 34. Sincecarriers (electrons) always travel in the same direction in the currentcontrol TFT, it is sufficient to provide the LDD region only on the sidecloser to the drain region.

From the view point of increasing a possible amount of current to flow,it is also effective to increase film thickness of the active layer (inparticular, a thickness at the channel forming region) of the currentcontrol TFT 202 (preferably in the range from 50 to 100 nm, and morepreferably in the range from 60 to 80 nm). On the other hand, in thecase of the switching TFT 201, from the view point of reducing an OFFcurrent value, it is also effective to decrease film thickness of theactive layer (in particular, a thickness at the channel forming region)of the current control TFT 202 (preferably in the range from 20 to 50μm, and more preferably in the range from 25 to 40 nm.)

Then, reference numeral 41 denotes the first passivation film, which mayhave a thickness of 10 μm to 1 μm (preferably 200 to 500 nm). As theconstituent material thereof, an insulating film containing silicon canbe used (particularly, a silicon oxynitride film or a silicon nitridefilm is preferred). It is effective to provide this passivation film 41with the heat radiation effect for preventing the EL layer from thermaldegradation.

A thin film exhibiting the heat radiation effect includes an insulatingfilm containing at least one element selected from B (boron), C(carbon), and N (nitrogen), as well as at least one element selectedfrom Al (aluminum), Si (silicon), and P (phosphorous). For example, itis possible to use a nitride of aluminum, typically aluminum nitride(AlxNy); a carbide of silicon, typically silicon carbide (SixCy); anitride of silicon, typically silicon nitride (SixNy); at, nitride ofboron, typically boron nitride (BxNy); and a phosphide of boron,typically boron phosphide (BxPy). Furthermore, an oxide of aluminum,typically aluminum oxide (AlxOy) has a thermal conductivity of 20Wm⁻¹K⁻¹, and can be one of preferable materials. In the above-mentionedtransparent materials, x and y can be any integer.

Note that it is also possible to combine the above compound with anotherelement. For example, it is also possible to use aluminum nitride oxideindicated by AlNxOy by adding nitrogen to the aluminum oxide. Note thatin the above aluminum nitride oxide, x and y are respectively arbitraryintegers.

Besides, it is possible to use materials disclosed in Japanese Laid-openPatent Publication No. Sho 62-90260. That is, it is also possible to usean insulating film containing Si, Al, N, O, or M (M is at least one kindof rare-earth element, preferably at least one element selected from Ce(cerium), Yb (ytterbium), Sm (samarium), Er (erbium), Y (yttrium), La(lantern), Gd (gadolinium), Dy (dysprosium), and Nd (neodymium)).

Besides, it is also possible to use a carbon film such as a diamond thinfilm or an amorphous carbon film (especially a film havingcharacteristics close to diamond, called diamond-like carbon or thelike). These have very high thermal conductivity and are very effectiveas a heat radiating layer.

Thus, although a thin film made of the material having the foregoingheat radiating effect can be used alone, it is effective to stack thesethin films and a silicon nitride film (SixNy) or silicon nitride oxidefilm (SiOxNy). Note that in the silicon nitride film or silicon nitrideoxide film, x and y are respectively arbitrary integers.

Over the first passivation film 41, a second interlayer insulating film42 (also referred to as a planarizing film) is formed to cover therespective TFTs, and thus steps caused by the TFTs are planarized. Asthe second interlayer insulating film 42, an organic resin film ispreferred, and polyimide, polyamide, acrylic, BCB (benzocyclobutane), orthe like can be used. An inorganic film ran be also used of course, solong as sufficient planarization is realized.

The planarization of the steps caused by the TFTs by means of the secondinterlayer insulating film 42 is very important. The EL layer to beformed in the subsequent step is so thin that the steps may cause todefect in light emission. Accordingly, in order to form the EL layer ona flat surface as much as possible, it is preferable to perform aplanarization process prior to the formation of the pixel electrode.

Reference numeral 43 denotes the pixel electrode (corresponding to thecathode of the EL element) made of a conductive film having a functionto shield light. The pixel electrode 43 is formed, after contact holes(openings) are provided in the second interlayer insulating film 42 andthe first passivation film 41, to be connected the drain wiring 37 ofthe current control TFT 202 in the thus formed opening portion.

A lithium fluoride film having a thickness of 5 to 10 nm is formed by avapor deposition method as the alkaline compound 44 on the pixelelectrode 43. The lithium fluoride film is an insulating film, and thus,when the thickness thereof is too large, a current cannot flow to the ELlayer. No adverse effect is generated even when the lithium fluoridefilm is formed in an island-like pattern, not in a layer.

The EL layer 45 is then formed. In the present embodiment, a polymertype organic material is formed through a spin coat technique. Any knownmaterial can be used as the polymer type organic material. Although asingle layer of the light emitting layer is used as the EL layer 45 inthe present embodiment, the laminated structure in which the lightemitting layer is combined with a hole transport layer, or an electrontransport layer provides a higher light emission efficiency. When thepolymer type organic material is to be laminated, it is preferable to becombined with a low molecule organic material formed by a vapordeposition technique. With the spin coat technique, if there is the baseorganic material, it may re-melt since the organic material to form theEL layer is mixed with an organic solvent and coated.

A typical polymer type organic material that can be used in the presentembodiment includes high molecule material such as polyparaphenylenevinylene (PPV) type, polyvinyl carbazole (PVK) type, polyfluorene type,or the like. In order to form an electron transport layer, a lightemitting layer, a hole transport layer or a hole injection layer withthese polymer type organic materials, the organic material may be coatedin the form of polymer precursor, and then heated (baked) in vacuum tobe converted into the polymer type organic material.

More specifically, as the polymer type organic material for providinglight of white color to be a light emitting layer, the materialsdisclosed in Japanese Laid-Open Patent Publication No. Hei 8-96959 orNo. Hei 9-63770 can be used. For example, the material obtained bysolving PVK (polyvinylcarbazole), Bu-PBD(2-(4′-tert-butylphenyl)-5-(4″-biphenyl)-1,3,4-oxydiazole), coumarin 6,DCM 1 (4-dicyanomethylene-2-methyl-6-p-dimethyl aminostilyl-4H-pyran),TPB (tetra phenyl butadiene), and Nile Red into 1,2-dichloromethane canbe used. The thickness of the above material can be set to be in therange from 30 to 150 nm (preferably 40 to 100 nm). As the hole transportlayer, polytetra hydrothiophenyl phenylene that is the polymerprecursor, is used, which is heated to be converted into polyphenylenevinylene. The thickness thereof can be set to be in the range from 30 to100 nm (preferably 40 to 80 nm).

Thus, the polymer type organic material is advantageous especially foremitting white light since color adjustment can be easily performed byadding a fluorescent pigment into a solution in which a host material issolved. Although the EL element is formed by using the polymer typeorganic material in the above description, any low molecule type organicmaterial can be used. Moreover, the EL layer can be formed with aninorganic material.

The above-mentioned organic materials are only examples that can be usedfor the EL layer in accordance with the present invention. Note that thepresent invention is not limited thereto.

When the EL layer 45 is formed, it is desirable that the process isproceeded in a dry inert gas atmosphere containing water as less aspossible. The EL layer is likely to easily deteriorate due to water oroxygen existing in the surrounding atmosphere, and therefore, thosefactors should be eliminated as much as possible in the formation of theEL layer. For example, a dry nitrogen atmosphere, a dry argonatmosphere, or the like is preferred. For that purpose, it is preferableto place a coating process chamber and a baking process chamber in aclean booth filled with inert gas and the process is proceeded in theabove-mentioned atmosphere.

After the EL layer 45 is formed in the above-mentioned manner, the anode46 made of a transparent conductive film, as well as the secondpassivation film 47, are formed. In the present embodiment, the anode 46is formed of a conductive film made of a compound of indium oxide andzinc oxide. A small amount of gallium may be added thereto. As thesecond passivation film 47, a silicon nitride film having a thickness of10 nm to 1 μm (preferably 200 to 500 nm) can be used.

Since the EL layer is vulnerable to heat as explained above, it isdesirable to deposit the anode 46 and the second passivation film 47 ata temperature of as low as possible (preferably in the range from roomtemperature to 120° C.). Accordingly, it can be said that a plasma CVDtechnique, a vacuum vapor deposition technique, or a solution coating(spin coat) technique is a preferred technique for the film formation.

The counter substrate 48 is disposed to face the active matrix substratethus completed. In the present embodiment, a glass substrate is used asthe counter substrate 48. Furthermore, the counter substrate 48 isprovided with light shielding films 49 a and 49 b made of resin with ablack-colored pigment being dispersed therein, and a color filter 50made of resin with a red-colored, green-colored, or blue-colored pigmentbeing dispersed therein. These light shielding films 49 a and 49 b aredisposed so as to conceal a gap between the pixel electrode 43 and itsadjacent pixel electrode. At this time, it is advantageous that thelight shielding films 49 a and 49 b contain a drying agent such asbarium oxide or the like. Other material such as those disclosed inJapanese Laid-Open Patent Publication No. Hei 9-148066 can be used asthe drying agent. Furthermore, the color filter 50 is formed at theposition corresponding to the pixel 102.

The active matrix substrate is adhered to the counter substrate 48 bymeans of a sealing agent (not illustrated) to form a closed space 51. Inthe present embodiment, the closed space is filled with argon gas. It isof course possible to place the above-mentioned drying agent in theclosed space 51.

The EL display device in accordance with the present embodiment includesa pixel portion composed of pixels each having the structure as shown inFIG. 2, in which the TFTs having the different structures in accordancewith their functions in pixels are arranged. More specifically, theswitching TFT having a sufficiently low OFF current value and thecurrent control TFT which is not vulnerable to the injection of hotcarriers are formed in the same pixel. Thus, the EL display device withhigh reliability capable of displaying an image with high definition canbe obtained.

Embodiment 1

The embodiments of the present invention are explained using FIGS. 4A to6C. A method of simultaneous manufacture of a pixel portion, and TFTs ofa driving circuit portion formed in the periphery of the pixel portion,is explained here. Note that in order to simplify the explanation, aCMOS circuit is shown as a basic circuit for the driving circuits.

First, as shown in FIG. 4A, a base film 301 is formed with a 300 nmthickness on a glass substrate 300. Oxidized silicon nitride films arelaminated as the base film 301 in embodiment 1. It is good to set thenitrogen concentration at between 10 and 25 wt % in the film contactingthe glass substrate 300.

Besides, as a part of the base film 301, it is effective to provide aninsulating film made of a material similar to the first passivation film41 shown in FIG. 2. The current controlling TFT is apt to generate heatsince a large current is made to flow, and it is effective to provide aninsulating film having a heat radiating effect at a place as close aspossible.

Next, an amorphous silicon film (not shown in the figures) is formedwith a thickness of 50 am on the base film 301 by a known depositionmethod. Note that it is not necessary to limit this to the amorphoussilicon film, and another film may be formed provided that it is asemiconductor film containing an amorphous structure (including amicrocrystalline semiconductor film). In addition, a compoundsemiconductor film containing an amorphous structure, such as anamorphous silicon germanium film, may also be used. Further, the filmthickness may be made from 20 to 100 nm.

The amorphous silicon film is then crystallized by a known method,forming a crystalline silicon film (also referred to as apolycrystalline silicon film or a polysilicon film) 302. Thermalcrystallization using an electric furnace, laser annealingcrystallization using a laser, and lamp annealing crystallization usingan infrared lamp exist as known crystallization methods. Crystallizationis performed in embodiment 1 using an excimer laser light which usesXeCl gas.

Note that pulse emission type excimer laser light formed into a linearshape is used in embodiment 1, but a rectangular shape may also be used,and continuous emission argon laser light and continuous emissionexcimer laser light can also be used.

In this embodiment, although the crystalline silicon film is used as theactive layer of the TFT, it is also possible to use an amorphous siliconfilm. Further, it is possible to form the active layer of the switchingTFT, in which there is a necessity to reduce the off current, by theamorphous silicon film, and to form the active layer of the currentcontrol TFT by the crystalline silicon film. Electric current flows withdifficulty in the amorphous silicon film because the carrier mobility islow, and the off current does not easily flow. In other words, the mostcan be made of the advantages of both the amorphous silicon film,through which current does not flow easily, and the crystalline siliconfilm, through which current easily flows.

Next, as shown in FIG. 4B, a protecting film 303 is formed on thecrystalline silicon film 302 with a silicon oxide film having athickness of 130 nm. This thickness may be chosen within the range of100 to 200 nm (preferably between 130 and 170 nm). Furthermore, otherfilms may also be used providing that they are insulating filmscontaining silicon. The protecting film 303 is formed so that thecrystalline silicon film is not directly exposed to plasma duringaddition of an impurity, and so that it is possible to have delicateconcentration control of the impurity.

Resist masks 304 a and 304 b are then formed on the protecting film 303,and an impurity element which imparts n-type conductivity (hereafterreferred to as an n-type impurity element) is added. Note that elementsresiding in periodic table group 15 are generally used as the n-typeimpurity element, and typically phosphorous or arsenic can be used. Notethat a plasma doping method is used, in which phosphine (PH₃) is plasmaactivated without separation of mass, and phosphorous is added at aconcentration of 1×10¹⁸ atoms/cm⁵ in embodiment 1. An ion implantationmethod, in which separation of mass is performed, may also be used, ofcourse.

The dose amount is regulated so that the n-type impurity element iscontained in n-type impurity regions 305 and 306, thus formed by thisprocess, at a concentration of 2×10¹⁶ to 5×10¹⁹ atoms/cm³ (typicallybetween 5×10¹⁷ and 5×10¹⁸ atoms/cm³).

Next, as shown in FIG. 4C, the protecting film 303 is removed, and anactivation of the added periodic table group 15 elements is performed. Aknown technique of activation may be used as the means of activation,but activation is done in embodiment 1 by irradiation of excimer laserlight. Of course, a pulse emission type excimer laser and a continuousemission type excimer laser may both, be used, and it is not necessaryto place any limits on the use of excimer laser light. The goal is theactivation of the added impurity element, and it is preferable thatirradiation is performed at an energy level at which the crystallinesilicon film does not melt. Note that the laser irradiation may also beperformed with the protecting film 303 in place.

The activation by heat treatment may also be performed along withactivation of the impurity element by laser light. When activation isperformed by heat treatment, considering the heat resistance of thesubstrate, it is good to perform heat treatment on the order of 450 to550° C.

A boundary portion (connecting portion) with end portions of the n-typeimpurity regions 305 and 306, namely regions, in which the n-typeimpurity element is not added, on the periphery of the n-type impurityregions 305 and 306, is not added, is delineated by this process. Thismeans that, at the point when the TFTs are later completed, extremelygood connections can be formed between LDD regions and channel formingregions.

Unnecessary portions of the crystalline silicon film are removed next,as shown in FIG. 4D, and island shape semiconductor films (hereafterreferred to as active layers) 307 to 310 are formed.

Then, as shown in FIG. 4E, a gate insulating film 311 is formed,covering the active layers 307 to 310. An insulating film containingsilicon and with a thickness of 10 to 200 nm, preferably between 50 and150 nm, may be used as the gate insulating film 311. A single layerstructure or a lamination structure may be used. A 110 nm thick oxidizedsilicon nitride film is used in embodiment 1.

Thereafter, a conductive film having a thickness of 200 to 400 nm isformed and patterned to form gate electrodes 312 to 316. Respective endportions of these gate electrodes 312 to 316 may be tapered. In thepresent embodiment, the gate electrodes and wirings (hereinafterreferred to as the gate wirings) electrically connected to the gateelectrodes for providing conducting paths are formed of differentmaterials from each other. More specifically, the gate wirings are madeof a material having a lower resistivity than the gate electrodes. Thus,a material enabling fine processing is used for the gate electrodes,while the gate wirings are formed of a material that can provide asmaller wiring resistance but is not suitable for fine processing. It isof course possible to form the gate electrodes and the gate wirings withthe same material.

Although the gate electrode can be made of a single-layered conductivefilm, it is preferable to form a lamination film with two, three or morelayers for the gate electrode if necessary. Any known conductivematerials can be used for the gate electrode. It should be noted,however, that it is preferable to use such a material that enables fineprocessing, and more specifically, a material that can be patterned witha line width of 2 μm or less.

Typically, it is possible to use a film made of an element selected fromtantalum (Ta), titanium (Ti), molybdenum (Mo), tungsten (W), chromium(Cr), and silicon (Si), a film of nitride of the above element(typically a tantalum nitride film, tungsten nitride film, or titaniumnitride film), an alloy film of combination of the above elements(typically Mo—W alloy, Mo—Ta alloy), or a silicide film of the aboveelement (typically a tungsten silicide film or titanium silicide film).Of course, the films may be used as a single layer or a laminate layer.

In this embodiment, a laminate film of a tungsten nitride (WN) filmhaving a thickness of 50 nm and a tungsten (W) film having a thicknessof 350 nm is used. This may be formed by a sputtering method. When aninert gas of Xe, Ne or the like is added as a sputtering gas, filmpeeling due to stress can be prevented.

The gate electrodes 313 and 316 are formed at this time so as to overlapa portion of the n-type impurity regions 305 and 306, respectively,sandwiching the gate insulating film 311. This overlapping portion laterbecomes an LDD region overlapping the gate electrode.

Next, an n-type impurity element (phosphorous is used in embodiment 1)is added in a self-aligning manner with the gate electrodes 312 to 316as masks, as shown in FIG. 5A. The addition is regulated so thatphosphorous is added to impurity regions 317 to 323 thus formed at aconcentration of 1/10 to ½ that of the impurity regions 305 and 306(typically between ¼ and ⅓). Specifically, a concentration of 1×10¹⁶ to5×10¹⁸ atoms/cm³ (typically 3×10¹⁷ to 3×10¹⁸ atoms/cm³) is preferable.

Resist masks 324 a to 324 c are formed next, with a shape covering thegate electrodes etc., as shown in FIG. 5B, and an n-type impurityelement (phosphorous is used in embodiment 1) is added, forming impurityregions 325 to 331 containing phosphorous at high concentration ofphosphorous. Ion doping using phosphine (PH₃) is also performed here,and is regulated so that the phosphorous concentration of these regionsis from 1×10²⁰ to 1×10²¹ atoms/cm³ (typically between 2×10²⁰ and 5×10²¹atoms/cm³).

A source region or a drain region of the n-channel type TFT is formed bythis process and in the switching TFT, a portion of the n-type impurityregions 320 to 322 formed by the process of FIG. 5A are remained. Theseremaining regions correspond to the LDD regions 15 a to 15 d of theswitching TFT in FIG. 2.

Next, as shown in FIG. 5C, the resist masks 324 a to 324 c are removed,and a new resist mask 332 is formed. A p-type impurity element (boron isused in embodiment 1) is then added, forming impurity regions 333 and334 containing boron at high concentration. Boron is added here to formimpurity regions 333 and 334 at a concentration of 3×10²⁰ to 3×10²¹atoms/cm³ (typically between 5×10²⁰ and 1×10²¹ atoms/cm³) by ion dopingusing diborane (B₂H₆).

Note that phosphorous has already been added to the impurity regions 333and 334 at a concentration of 1×10²⁰ to 1×10²¹ atoms/cm³, but boron isadded here at a concentration of at least 3 times that of thephosphorous. Therefore, the n-type impurity regions already formedcompletely invert to p-type, and function as p-type impurity regions.

Next, after removing the resist mask 332, the n-type and p-type impurityelements added to the active layer at respective concentrations areactivated. Furnace annealing, laser annealing or lamp annealing can beused as a means of activation. In embodiment 1, heat treatment isperformed for 4 hours at 550° C. in a nitrogen atmosphere in an electricfurnace.

At this time, it is critical to eliminate oxygen from the surroundingatmosphere as much as possible. This is because when even only a smallamount of oxygen exists, an exposed surface of the gate electrode isoxidized, which results in an increased resistance and later makes itdifficult to form an ohmic contact with the gate electrode. Accordingly,the oxygen concentration in the surrounding atmosphere for theactivation process is set at 1 ppm or less, preferably at 0.1 ppm orless.

After the activation process is completed, the gate wiring 335 having athickness of 300 nm is formed. As a material for the gate wiring 335, ametal film containing aluminum (Al) or copper (Cu) as its main component(occupied 50 to 100% in the composition) can be used. The gate wiring335 is arranged, as the gate wiring 211 shown in FIG. 3A, so as toprovide electrical connection for the gate electrodes 314 and 315(corresponding to the gate electrodes 19 a and 19 b in FIG. 3A) of theswitching TFT (see FIG. 5D).

The above-described structure can allow the wiring resistance of thegate wiring to be significantly reduced, and therefore, an image displayregion (pixel portion) with a large area can be formed. Morespecifically, the pixel structure in accordance with the presentembodiment is advantageous for realizing an EL display device having adisplay screen with a diagonal size of 10 inches or larger (or 30 inchesor larger.)

A first interlayer insulating film 336 is formed next, as shown in FIG.6A. A single layer insulating film containing silicon is used as thefirst interlayer insulating film 336, while a lamination film may beused. Further, a film thickness of between 400 nm and 1.5 μm may beused. A lamination structure of an 800 nm thick silicon oxide film on a200 nm thick oxidized silicon nitride film is used in embodiment 1.

In addition, heat treatment is performed for 1 to 12 hours at 300 to450° C. in an atmosphere containing between 3 and 100% hydrogen,performing hydrogenation. This process is one of hydrogen termination ofdangling bonds in the semiconductor film by hydrogen which is thermallyactivated. Plasma hydrogenation (using hydrogen activated by a plasma)may also be performed as another means of hydrogenation.

Note that the hydrogenation processing may also be inserted during theformation of the first interlayer insulating film 336. Namely, hydrogenprocessing may be performed as above after forming the 200 nm thickoxidized silicon nitride film, and then the remaining 800 nm thicksilicon oxide film may be formed.

Next, a contact hole is formed in the first interlayer insulating film336, and source wiring lines 337 to 340 and drain wiring lines 341 to343 are formed. In this embodiment, this electrode is made of a laminatefilm of three-layer structure in which a titanium film having athickness of 100 nm, an aluminum film containing titanium and having athickness of 300 nm, and a titanium film having a thickness of 150 nmare continuously formed by a sputtering method. Of course, otherconductive films may be used.

A first passivation film 344 is formed next with a thickness of 50 to500 nm (typically between 200 and 300 nm). A 300 nm thick oxidizedsilicon nitride film is used as the first passivation film 344 inembodiment 1. This may also be substituted by a silicon nitride film. Itis of course possible to use the same materials as those of the firstpassivation film 41 of FIG. 2.

Note that it is effective to perform plasma processing using a gascontaining hydrogen such as H₂ or NH₃ etc. before the formation of theoxidized silicon nitride film. Hydrogen activated by this preprocess issupplied to the first interlayer insulating film 336, and the filmquality of the first passivation film 344 is improved by performing heattreatment. At the same time, the hydrogen added to the first interlayerinsulating film 336 diffuses to the lower side, and the active layerscan be hydrogenated effectively.

Next, as shown in FIG. 6B, a second interlayer insulating film 345 madeof organic resin is formed. As the organic resin, it is possible to usepolyimide, polyamide, acryl, BCB (benzocyclobutene) or the like.Especially, since the second interlayer insulating film 345 is primarilyused for flattening, acryl excellent in flattening properties ispreferable. In this embodiment, an acrylic film is formed to a thicknesssufficient to flatten a stepped portion formed by TFTs. It isappropriate that the thickness is made 1 to 5 μm (more preferably, 2 to4 μm).

Thereafter, a contact hole is formed in the second interlayer insulatingfilm 345 and the first passivation film 344 to reach the drain wiring343, and then the pixel electrode 346 is formed. In the presentembodiment, an aluminum alloy film (an aluminum film containing titaniumof 1 wt %) having a thickness of 300 nm is formed as the pixel electrode346. Reference numeral 347 denotes an end portion of the adjacent pixelelectrode.

Then, the alkaline compound 348 is formed, as shown in FIG. 6C. In thepresent embodiment, a lithium fluoride film is formed by a vapordeposition method so as to have a film thickness of 5 nm. Thereafter,the EL layer 349 having a thickness of 100 nm is formed by spin coating.

In the present embodiment, as the polymer type organic material forproviding light of white color, the materials disclosed in JapaneseLaid-Open Patent Publication No. Hei 8-96959 or No. Hei 9-63770 can beused. For example, the material obtained by solving PVK (polyvinylcarbazole), Bu-PBD(2-(4′-tert-butylphenyl)-5-(4″-biphenyl)-1,3,4-oxydiazole), coumarin 6,DCM 1 (4-dicyanomethylene-2-methyl-6-p-dimethyl aminostilyl-4H-pyran),TPB (tetra phenyl butadiene), and Nile Red into 1,2-dichloromethane canbe used.

In the present embodiment, the EL layer 349 has a single layer structureincluding only the above-mentioned light emitting layer. Alternatively,an electron injection layer, an electron transport layer, a holetransport layer, a hole injection layer, an electron blocking layer, ora hole element layer can be further formed, if necessary.

Then, the anode 350 made of a transparent conductive film having athickness of 200 nm is formed to cover the EL layer 349. In thisembodiment, a film made of a compound of indium oxide and zinc oxide isformed by a vapor deposition and then patterned to obtain the anode.

Finally, the second passivation film 351 made of a silicon nitride filmis formed by a plasma CVD to have a thickness of 100 nm. This secondpassivation film 351 is intended to provide protection for the EL layer349 against water or the like, and also function to release heatgenerated in the EL layer 349. In order to further enhance the heatradiation effect, it is advantageous to form the second passivation filmby forming a silicon nitride film and a carbon film (preferably adiamond-like carbon film) into the lamination structure.

In this way, an active matrix EL display device having a structure asshown in FIG. 6C is completed. In the active matrix EL display device ofthis embodiment, a TFT having an optimum structure is disposed in notonly the pixel portion but also the driving circuit portion, so thatvery high reliability is obtained and operation characteristics can alsobe improved.

First, a TFT having a structure to decrease hot carrier injection so asnot to drop the operation speed thereof as much as possible is used asan n-channel type TFT 205 of a CMOS circuit forming a driving circuit.Note that the driving circuit here includes a shift register, a buffer,a level shifter, a sampling circuit (sample and hold circuit) and thelike. In the case where digital driving is made, a signal conversioncircuit such as a D/A converter can also be included.

In the case of this embodiment, as shown in FIG. 6C, the active layer ofthe n-channel TFT 205 includes a source region 355, a drain region 356,an LDD region 357 and a channel forming region 358, and the LDD region357 overlaps with the gate electrode 313 through the gate insulatingfilm 311.

Consideration not to drop the operation speed is the reason why the LDDregion is formed at only the drain region side. In this n-channel typeTFT 205, it is not necessary to pay attention to an off current valuevery much, rather, it is better to give importance to an operationspeed. Thus, it is desirable that the LDD region 357 is made tocompletely overlap with the gate electrode to decrease a resistancecomponent to a minimum. That is, it is preferable to remove theso-called offset.

Furthermore, deterioration of the p-channel type TFT 206 in the CMOScircuit due to the injection of hot carriers is almost negligible, andthus, it is not necessary to provide any LDD region for the p-channeltype TFT 206. It is of course possible to provide the MD region for thep-channel type TFT 206, similarly for then-channel type TFT 205, toexhibit countermeasure against the hot carriers.

Note that, among the driving circuits, the sampling circuit is somewhatunique compared to the other circuits, in that a large electric currentflows in both directions in the channel forming region. Namely, theroles of the source region and the drain region are interchanged. Inaddition, it is necessary to control the value of the off current to beas small as possible, and with that in mind, it is preferable to use aTFT having functions which are on an intermediate level between theswitching TFT and the current control TFT in the sampling circuit. Acombination of an n-channel type TFT 207 and a p-channel type TFT 208 asshown in FIG. 10 is used as the sampling circuit in embodiment 1.

Accordingly, in the n-channel type TFT for forming the sampling circuit,it is desirable to arrange the TFTs having the structure as shown inFIG. 10. As illustrated in FIG. 10, portions of the LDD regions 901 aand 901 b overlap the gate electrode 903 through the gate insulatingfilm 902. The advantages obtainable by this structure have been alreadydescribed with respect to the current control TFT 202. In the case wherethe TFT is used for the sampling circuit, the LDD regions are disposedto interpose the channel forming region 904 therebetween, which isdifferent from the case of the current control TFT.

In the actual process, after the structure shown in FIG. 6C iscompleted, the EL layer is sealed in the closed space by using thecounter substrate provided with the light shielding film, as previouslydescribed with reference to FIGS. 1 and 2. At this time, the reliability(lifetime) of the EL layer can be improved by setting an inertatmosphere within the closed space or disposing a moisture absorbingmaterial (e.g., barium oxide) in the closed space. Such a sealingprocess of the EL layer can be performed by using the technique to beused in the cell assembly step for liquid crystal display devices.

After the sealing process of the EL layer is completed, a connector(flexible print circuit: FPC) is attached for connecting the terminalsextended from the elements or circuits formed on the substrate toexternal signal terminals, thereby completing a final product.

Here, the structure of the active matrix EL display device of thisembodiment will be described with reference to a perspective view ofFIG. 7. The active matrix EL display device of this embodiment isconstituted by a pixel portion 602, a gate side driving circuit 603, anda source side driving circuit 604 formed on a glass substrate 601. Aswitching TFT 605 of a pixel portion is an n-channel type TFT, and isdisposed at an intersection point of a gate wiring line 606 connected tothe gate side driving circuit 603 and a source wiring line 607 connectedto the source side driving circuit 604. The drain of the switching TFT605 is connected to the gate of a current control TFT 608.

Furthermore, the source side of the current control TFT 608 is connectedto the power supply line 609. In the structure in accordance with thepresent embodiment, the power supply line 609 is connected to the sourceof the EL element 610, and the drain of the current control TFT 608 isconnected to the EL element 610.

If the current control TFT 608 is an n-channel type TFT, then a cathodeof the EL element 610 is electrically connected to the drain. Further,in a case of using a p-channel type TFT for the current control TFT 608,an mode of the EL element 610 is electrically connected to the drain.

Input/output wiring lines (connection wiring lines) 612 and 613 fortransmitting signals to the driving circuits and a connection wiringline 614 connected to the current supply line 609 are provided in an FPC611 as an external input/output terminal.

An example of circuit structure of the EL display device shown in FIG. 7is shown in FIG. 8. The EL display device of this embodiment includes asource side driving circuit 701, a gate side driving circuit (A) 707, agate side driving circuit (B) 711, and a pixel portion 706. Note that inthe present specification, the term driving circuit is a general tendincluding the source side driving circuit and the gate side drivingcircuit.

The source side driving circuit 701 is provided with a shift register702, a level shifter 703, a buffer 704, and a sampling circuit (sampleand hold circuit) 705. The gate side driving circuit (A) 707 is providedwith a shift register 708, a level shifter 709, and a buffer 710. Thegate side driving circuit (B) 711 also has the same structure.

Here, the shift registers 702 and 708 have driving voltages of 5 to 16 V(typically 10 V) respectively, and the structure indicated by 205 inFIG. 6C is suitable for an n-channel type TFT used in a CMOS circuitforming the circuit.

Besides, for each of the level shifters 703 and 709 and the buffers 704and 710, similarly to the shift register, the CMOS circuit including then-channel type TFT 205 of FIG. 6C is suitable. Note that it is effectiveto make a gate wiring line a multi-gate structure such as a double gatestructure or a triple gate structure in improving reliability of eachcircuit.

Besides, since the source region and the drain region are inverted andit is necessary to decrease an off current value, a CMOS circuitincluding the n-channel type TFT 208 of FIG. 10 is suitable for thesampling circuit 705.

The pixel portion 706 is disposed with pixels having the structure shownin FIG. 2.

The foregoing structure can be easily realized by manufacturing TFTs inaccordance with the manufacturing steps shown in FIGS. 4A to 6C. In thisembodiment, although only the structure of the pixel portion and thedriving circuit is shown, if the manufacturing steps of this embodimentare used, it is possible to form a logical circuit other than thedriving circuit, such as a signal dividing circuit, a D/A convertercircuit, an operational amplifier circuit, a {hacek over (a)}-correctioncircuit, or the like on the same substrate, and further, it isconsidered that a memory portion, a microprocessor, or the like ran beformed.

Furthermore, the EL display device in accordance with the presentembodiment will be described with reference to FIGS. 11A and 11B. Thereference signs used in FIGS. 7 and 8 are referred if necessary.

A substrate 1000 (including an base film beneath TFTs) is an activematrix substrate. On the substrate, a pixel portion 1001, a source sidedriving circuit 1002, and a gate side driving circuit 1003 are formed.Various wirings from the respective driving circuits are extendedthrough connection wirings 612 to 614 to reach an FPC 611 and beconnected to an external device.

At this time, a counter substrate 1004 is provided to surround at leastthe pixel portion, and more preferably, the driving circuits and thepixel portion. The counter substrate 1004 is adhered to the activematrix substrate 1000 by means of an adhesive (sealing agent) 1005 toform a closed space 1006. Thus, the EL element is completely sealed inthe closed space 1006 and shut out from the external air.

In the present embodiment, a photocurable epoxy resin is used as theadhesive 1005. Alternatively, other adhesives such as an acrylate typeresin can also be used. A thermosetting resin can be also used ifacceptable in view of heat-resistance of the EL element. Note that thematerial is required to prevent oxygen and water from passingtherethrough as much as possible. The adhesive 1005 can be applied by acoating device such as a dispenser:

Furthermore, in the present embodiment, the closed space 1006 betweenthe counter substrate 1004 and the active matrix substrate 1000 is filedwith nitrogen gas. Moreover, the counter substrate 1004 is provided onits inner side (on the side closer to the closed space) with a lightshielding film 1007 and a color filter 1008, as described with referenceto FIGS. 1 and 2. In the present embodiment, a resin film containingbarium oxide and a black-colored pigment is used as the light shieldingfilm 1007, and a resin film containing a red-colored, green-colored, orblue-colored pigment can be used as the color filter 1008.

Furthermore, as shown in FIG. 11B, the pixel portion is pi vided with aplurality of pixels each including an individually separated EL element.All of these EL elements share an anode 1009 as a common electrode. TheEL layer may be provided only in the pixel portion, but is not requiredto be disposed over the driving circuits. In order to selectivelyprovide the EL layer, a vapor deposition method employing a shadow mask,a lift-off method, a dry etching method, or a laser scribing method canbe used.

The anode 1009 is electrically connected to a connection wiring 1010.The connection wiring 1010 is a power supply line to be used forsupplying a predetermined voltage to the anode 1009, and is connected tothe FPC 611 through a conductive paste material 1011. Although only theconnection wiring 1010 is described herein, the other connection wirings612 to 614 are also electrically connected to the FPC 611 in the similarmanner.

As described above, the structure as shown in FIGS. 11A and 11B candisplay an image on its pixel portion by connecting the FPC 611 to aterminal of an external device. In the present specification, the ELdisplay device is defined as a product in which an image display becomespossible when an FPC is attached thereto, in other words, a productobtained by attaching an active matrix substrate to a counter substrate(including the one provided with an FPC attached thereto.)

Embodiment 2

In this embodiment, an example in which a structure of a pixel is madedifferent from the structure shown in FIG. 3B will be described withreference to FIG. 12. In this embodiment, two pixels shown in FIG. 3Bare arranged to become symmetrical with respect to a current supply line212 which applies ground electric potential. That is, as shown in FIG.12, a current supply line 213 is made common to two adjacent pixels, sothat the number of necessary wiring lines can be reduced. Incidentally,a TFT structure or the like arranged in the pixel may remain the same.

If such structure is adopted, it becomes possible to manufacture a moreminute pixel portion, and the quality of an image is improved.

Incidentally, the structure of this embodiment can be easily realized inaccordance with the manufacturing steps of the embodiment 1, and withrespect to the TFT structure or the like, the description of theembodiment 1 or FIG. 2 may be referred to.

Embodiment 3

Although the description has been made on the case of the top gate typeTFT in the embodiments 1 and 2, the present invention is not limited tothe TFT structure, and may be applied to a bottom gate type TFT(typically, inverted stagger type TFT). Besides, the inverted staggertype TFT may be formed by any means.

Since the inverted stagger type TFT has such a structure that the numberof steps can be easily made smaller than the top gate type TFT, it isvery advantageous in reducing the manufacturing cost, which is theobject of the present invention. Incidentally, the structure of thisembodiment can be freely combined with any structure of the embodiments2 and 3.

Embodiment 4

FIG. 3B shows that the amount of the off current value in the switchingTFT in the pixel of the EL display device is reduced by using amulti-gate structure for the switching TFT, and the need for a storagecapacitor is eliminated. However, it is also acceptable to make astructure of disposing a storage capacitor as is done conventionally. Inthis case, as shown in FIG. 14, a storage capacitor 1301 is formed inparallel to the gate of the current control TFT 202 with respect to thedrain of the switching TFT 201.

Note that the constitution of embodiment 4 can be freely combined withany constitution of embodiments 1 to 3. Namely, a storage capacitor ismerely formed within a pixel and it is not to limit the TFT structure,materials of EL layer, etc.

Embodiment 5

Laser crystallization is used as the means of forming the crystallinesilicon film 302 in embodiment 1, and a case of using a different meansof crystallization is explained in embodiment 5.

After forming an amorphous silicon film in embodiment 5, crystallizationis performed using the technique recorded in Japanese Patent ApplicationLaid-open No. Hei 7-130652. The technique recorded in the above patentapplication is one of obtaining a crystalline silicon film having goodcrystallinity by using an element such as nickel as a catalyst forpromoting crystallization.

Further, after the crystallization process is completed, a process ofremoving the catalyst used in the crystallization may be performed. Inthis case, the catalyst may be gettered using the technique recorded inJapanese Patent Application Laid-open No. Hei 10-270363 or JapanesePatent Application Laid-open No. Hei 8-330602.

In addition, a may be formed using the technique recorded in thespecification of Japanese Patent Application No. Hei 11-076967 by theapplicant of the present invention.

The processes of manufacturing shown in embodiment 1 are one embodimentof the present invention, and provided that the structure of FIG. 1 orof FIG. 6C of embodiment 1 can be realized, then other manufacturingprocess may also be used without any problems, as above.

Note that it is possible to freely combine the constitution ofembodiment 5 with the constitution of any of embodiments 1 to 4.

Embodiment 6

In driving the EL display device of the present invention, analogdriving can be performed using an analog signal as an image signal, anddigital driving can be performed using a digital signal.

When analog driving is performed, the analog signal is sent to a sourcewiring of a switching TFT, and the analog signal, which contains grayscale information, becomes the gate voltage of a current control TFT.The current flowing in an EL element is then controlled by the currentcontrol TFT, the EL element emitting intensity is controlled, and grayscale display is performed.

On the other band, when digital driving is performed, it differs fromthe analog type gray scale display, and gray scale display is performedby time division driving.

The EL element has an extremely fast response speed in comparison to aliquid crystal element, and therefore it is possible to have high speeddriving. Therefore, the EL element is one, which is suitable for timeratio gray scale driving, in which one frame is partitioned into aplural number of subframes and then gray scale display is performed.

The present invention is a technique related to the element structure,and therefore any method of driving it may thus be used.

Embodiment 7

The EL display device uses light emitted from itself, and thus, does notrequire any back light. A reflection type liquid crystal display devicerequires a back light in a dark place where sufficient light is notavailable, although it has a feature in that an image can be displayedwith outdoor light. On the other hand, the EL display device is notsuffered from such a disadvantage in a dark place, since it is of theself-emission type.

However, when an electronic device including the EL display device asits display portion is actually used outdoors, it may be of course usedboth in a light place and in a dark place. In such a situation, an imagecan be sufficiently recognized in a dark place even when the luminanceis not so high, while an image may not be recognized in a light place ifthe luminance is not sufficiently high.

An amount of light emitted from the EL layer varies depending on anamount of current to flow. Thus, a larger amount of current to flowrequires the higher luminance, resulting in an increased powerconsumption. However, when the luminance of emitted light is set at sucha high level, too brighter image than necessary with too large powerconsumption will be displayed in a dark place.

In order to overcome the above-mentioned disadvantage, the EL displaydevice in accordance with the present invention preferably has afunction to detect the lightness in the surrounding atmosphere by meansof a sensor, and adjust the luminance of the light emitted from the ELlayer in accordance with the sensed lightness. More specifically, theluminance of the emitted light is set at a high level in a light place,while at a low level in a dark place, so that an increase in powerconsumption is avoided. Thus, the EL display device in accordance withthe present invention can realize reduction in power consumption.

As a sensor to be used for detecting lightness in the surroundingatmosphere, a CMOS sensor, a CCD or the like can be used. A CMOS sensorcan be formed with any known technique on the identical substrate withdriving circuits and a pixel portion of the EL display device. Asemiconductor chip on which a CCD is formed can be attached onto the ELdisplay device. Alternatively, a CCD or a CMOS sensor may be provided asa portion of an electronic device including the EL display device as itsdisplay portion.

A circuit for adjusting a current to flow into the EL layer based on asignal obtained by the sensor for detecting the lightness in thesurrounding atmosphere is provided. Thus, the luminance of the lightemitted from the EL layer can be adjusted in accordance with thelightness in the surrounding atmosphere.

The structure in the present embodiment is applicable in combinationwith any structure in Embodiment 1 through 6.

Although the preferred embodiments of the present invention utilize thinfilm transistors as switching elements formed over an insulatingsubstrate, it is possible to utilize a silicon substrate. In this case,insulated gate field effect transistors formed with the siliconsubstrate can be used as switching elements.

Embodiment 8

The EL display device fabricated in accordance with the presentinvention is of the self-emission type, and thus exhibits more excellentrecognizability of the displayed image in a light place as compared tothe liquid crystal display device. Furthermore, the EL display devicehas a wider viewing angle. Accordingly, the EL display device can beapplied to a display portion in various electronic devices. For example,in order to view a TV program or the like on a large-sized screen, theEL display device in accordance with the present invention can be usedas a display portion of an EL display (i.e., a display in which an ELdisplay device is installed into a frame) having a diagonal size of 30inches or larger (typically 40 inches or larger.)

The EL display includes all kinds of displays to be used for displayinginformation, such as a display for a personal computer, a display forreceiving a TV broadcasting program, a display for advertisementdisplay. Moreover, the EL display device in accordance with the presentinvention can be used as a display portion of other various electricdevices.

Such electronic devices include a video camera, a digital camera, agoggles-type display (head mount display), a car navigation system, acar audio equipment, note-size personal computer, a game machine, aportable information terminal (a mobile computer, a portable telephone,a portable game machine, an electronic book, or the like), an imagereproduction apparatus including a recording medium (more specifically,an apparatus which can reproduce a recording medium such as a compactdisc (CD), a laser disc (LD), a digital video disc (DVD), and includes adisplay for displaying the reproduced image), or the like. Inparticular, in the case of the portable in formation terminal, use ofthe EL display device is preferable, since the portable informationterminal that is likely to be viewed from a tilted direction is oftenrequired to have a wide viewing angle. FIGS. 14A to 14F respectivelyshow various specific examples of such electronic devices.

FIG. 14A illustrates an EL display which includes a frame 2001, asupport table 2002, a display portion 2003, or the like. The presentinvention is applicable to the display portion 2003. The EL display isof the self-emission type and therefore requires no back light. Thus,the display portion thereof can have a thickness thinner than that ofthe liquid crystal display device.

FIG. 14B illustrates a video camera which includes a main body 2101, adisplay portion 2102, an audio input portion 2103, operation switches2104, a battery 2105, an image receiving portion 2106, or the like. TheEL display device in accordance with the present invention can be usedas the display portion 2102.

FIG. 14C illustrates a portion (the right-half piece) of an EL displayof head mount type, which includes a main body 2201, signal cables 2202,a head mount band 2203, a display portion 2204, an optical system 2205,an EL display device 2206, or the Eke. The present invention isapplicable to the EL display device 2206.

FIG. 14D illustrates an image reproduction apparatus including arecording medium (more specifically, a DVD reproduction apparatus),which includes a main body 2301, a recording medium (a CD, an LD, a DVDor the like) 2302, operation switches 2303, a display portion (a) 2304,another display portion (b) 2305, or the like. The display portion (a)is used mainly for displaying image information, while the displayportion (b) is used mainly for displaying character information. The ELdisplay device in accordance with the present invention can be used asthese display portions (a) and (b). The image reproduction apparatusincluding a recording medium further includes a CD reproductionapparatus, a game machine or the like.

FIG. 14E illustrates a portable (mobile) computer which includes a mainbody 2401, a camera portion 2402, an image receiving portion 2403,operation switches 2404, a display portion 2405, or the like. The ELdisplay device in accordance with the present invention can be used asthe display portion 2405.

FIG. 14F illustrates a personal computer which includes a main body2501, a frame 2502, a display portion 2503, a key board 2504, or thelike. The EL display device in accordance with the present invention canbe used as the display portion 2503.

When the brighter luminance of light emitted from the EL materialbecomes available in the future, the EL display device in accordancewith the present invention will be applicable to a front-type orrear-type projector in which light including output image information isenlarged by means of lenses or the like to be projected.

The aforementioned electronic devices are more likely to be used fordisplay information distributed through a telecommunication path such asInternet, a CATV (cable television sys tem), and in particular likely todisplay moving picture information. The EL display device is suitablefor displaying moving pictures since the EL material can exhibit highresponse speed. However, if the contour between the pixels becomesunclear, the moving pictures as a whole cannot be clearly displayed.Since the EL display device in accordance with the present invention canmake the contour between the pixels clear, it is significantlyadvantageous to apply the EL display device of the present invention toa display portion of the electronic devices.

A portion of the EL display device that is emitting light consumespower, so it is desirable to display information in such a manner thatthe light emitting portion therein becomes as small as possible.Accordingly, when the EL display device is applied to a display portionwhich mainly displays character information, e.g., a display portion ofa portable information terminal, and more particular, a portabletelephone or a car audio equipment, it is desirable to drive the ELdisplay device so that the character information is formed by alight-emitting portion while a non-emission portion corresponds to thebackground.

With now reference to FIG. 15A, a portable telephone is illustrated,which includes a main body 2601, an audio output portion 2602, an audioinput portion 2603, a display portion 2604, operation switches 2605, andan antenna 2606. The EL display device in accordance with the presentinvention can be used as the display portion 2604. The display portion2604 can reduce power consumption of the portable telephone bydisplaying white-colored characters on a black-colored background.

FIG. 15B illustrates a car audio equipment which includes a main body2701, a display portion 2702, and operation switches 2703 and 2704. TheEL display device in accordance with the present invention can be usedas the display portion 2702. Although the car audio equipment of themount type is shown in the present embodiment, the present invention isalso applicable to a car audio of the set type. The display portion 2702can reduce power consumption by displaying white-colored characters on ablack-colored background, which is particularly advantageous for the caraudio of the set type.

As set forth above, the present invention can be applied variously to awide range of electronic devices in all fields. The electronic device inthe present embodiment can be obtained by utilizing an EL display devicehaving the configuration in which the structures in Embodiments 1through 7 are freely combined.

In accordance with the present invention, in the pixel portion of the ELdisplay device, the contour between the pixels can be made clear, andthe EL display device capable of displaying an image with highdefinition can be provided. Moreover, in the present invention the lightshielding film to be used for concealing the gaps between the pixels isprovided on the counter substrate, thereby preventing yield fromdecreasing. Furthermore, the EL display device in accordance with thepresent invention can be fabricated by using a manufacturing line forliquid crystal display devices, and thus, equipment investment can besuppressed to a small level. Accordingly, in accordance with the presentinvention, an inexpensive EL display device capable of displaying animage with high definition ran be obtained. Furthermore, the presentinvention can also provide an electronic device having a display portionwith a high recognizability by utilizing such an EL display device asthe display portion.

What is claimed is:
 1. An EL display device, comprising: an activematrix substrate on which a plurality of pixels are arranged, each ofthe pixels including a TFT, a pixel electrode electrically connected tothe TFT, and an EL element including the pixel electrode as a cathode;and a counter substrate opposed to the active matrix substrate, whereina closed space is provided between the active matrix substrate and thecounter substrate attached to each other, and the counter substrate isprovided with a light shielding film disposed at positions respectivelycorresponding to peripheries of the respective pixels on theactive-matrix substrate.